1. Technical Field of the Invention
The present invention relates generally to a multi-level power converter designed to change one of a plurality of dc voltage inputs into an ac voltage output.
2. Background of Related Art
Japanese Patent First Publication No. 5-308778 discloses a conventional multi-level inverter.
FIG. 8 shows a circuit (e.g., a U-phase circuit) in one of three phases (U-phase, V-phase, and W-phase) of a conventional four-level inverter designed to provide a three-phase ac voltage. The circuit includes dc power supplies 101 to 104 that have dc voltages of different levels at dc voltage terminals 101a to 105a. Between the dc voltage terminals 101a to 105a and an U-phase output terminal 106, U-phase inverter arms are disposed, respectively, which include switching devices 111a to 118a such as MOS transistors connected in series, flywheel diodes 111b to 118b connected backward in parallel to the switching elements 111a to 118a, and diodes 121 to 125 connected to dc voltage dividing points, respectively. The output ac voltage from the output terminal 106 is achieved by subjecting the switching elements 111a to 118a selectively to PWM (Pulse Width Modulation) control.
V- and W-phase circuits are connected to the same dc power supplies 101 to 104 as used in the above described U-phase circuit and include the same inverter arms as those of the U-phase circuit, respectively. The output terminals of the U-, V-, and W-phase circuits provide a three-phase ac voltage to an electric load such as a three-phase ac motor.
FIG. 9 shows a drive circuit for applying the voltage across a gate and a source of each of the switching elements 111a to 118a. The drive circuit has a DC/DC converter 131 which consists of a DC/AC converter 1311, a transformer 1312, and AC/DC converters 1313 to 1320 which convert dc voltage from a dc power supply 130 into floating voltages and outputs them.
The AC/DC converters 1313 to 1320 are connected through drivers 141 to 148 to the switching elements 111a to 118a. Specifically, terminals (1) to (16) in FIG. 9 are connected to terminals (1) to (16) in FIG. 8, respectively.
The floating voltages outputted from the AC/DC converters 1313 to 1320 are applied through drivers 141 to 148 across the gates and sources of the switching elements 111a to 118a, respectively.
The above described multi-level inverter, however, encounters drawbacks in that because of the switching elements 111a to 118a, are connected serially the diodes 121 to 125 are required to change the level of the output voltage, thus increasing the component parts of the inverter and the number of that require floating voltages being applied to the switching elements 111a to 118a that require floating power supplies (i.e., the AC/DC converters 1313 to 1320) which are identical in number with the switching elements 111a to 118a.